The glasses employed were subjected to careful spectroscopic examination, and to colour-measurement by the tintometer, but neither were found to give precise indications as to the protective power of the glasses, which is no doubt due to the absorption of the violet, and especially of the invisible ultra-violet rays.

"For example," he resumed, "the Lovibond tintometer tells me its story of the colour of the ink used in the ribbon of the machine that wrote this note as well as several standard specimens which I have been able to obtain from three machines on which it might have been written.